I like nature documentaries. PBS Nature is amazing. Their "Bears of the Last Frontier" is fascinating.
But, if I watch documentaries, I will usually choose a feature. A week ago, I went to a film festival that only features documentaries. I saw "The Final Member", which is about a man in Iceland who collects penises. I also saw "Searching for Sugarman", which is an inspiring movie about a humble musician.
I'll watch documentaries about all sorts of topics ranging from murder to pop culture to food and so on.